Thanks for your reply. What's odd about this is that when I moved into the house in 1996, they were there and they thrived for years. Then a friend gave me yellow/gold varieties, plus I've bought other colors. I thought for sure digging them all up last year would have solved the problem, but still no blooms.

In one bed, the rhizomes are covered with leaves, the top of the rhizome is above the soil. Perhaps I should keep the leaves off the tops?

It's odd that the yellow ones bloom, no others (i.e., not only purple, which are the majority of the others, but I've bought black, apricot colored, etc. too, only yellow/gold are blooming, and they have leaves on top for mulch).

Susan,I haven't noticed any significant difference in blooming rates in varieties I have in equal conditions. I do have a neighbor that has irises on the north side of her garage and she has ended up with more white than purple (the only colors she has, now). I'm not sure what she started with originally, they've been there over 14 years that I've seen.

Where I have irises, the ones in full sun bloom the most reliably (the purples are some of the ones in the most sun). Mine are planted shallow and have a fairly heavy leaf mulch over them. I don't know if this will help, but that's the best I've got, lol!